This is very understandable. If you don't do this, you can use the data of the relative universe directly. Look at the navigation output. You stand here and do not move. After 12 hours, the navigator tells you that you are now heading toward the "down" (in fact, according to your station The latitude is different, not necessarily the head down), which will make users feel confused.
On the displacement, the relative displacement data of the universe will be loyal to the earth's rotation, which is really sitting on the ground for 80,000 miles. And what you want to know is just how much you went east and how much went north. Where is the north now? Where is the next step? In addition, in reality, the acceleration sensor cannot distinguish between acceleration and gravitation. When doing inertial navigation calculations, it is necessary to compensate for the interference of gravity.
So we need to convert the data of the inertial system into the data of the navigation system (generally the geographic system, that is, the northeastern day), that is, to subtract the earth's rotation, and the angular change brought by your latitude and longitude changes on the earth. This process, in the platform-based INS, is implemented by a physical platform that always tracks the location of the northeast, and is implemented by a series of formulas and calculations in the strapdown INS.